Heavy metals, such as lead, arsenic, cadmium, copper, and zinc, exhibit differing levels of toxicity within mammals. Particularly when ingested and in a state that can be metabolized and absorbed in the body, i.e., when it is ingested in a bioavailable form, such metals exhibit a propensity for toxic effects, even in low amounts, and upon accumulation within the body. As a result, regulatory agencies at both the state and federal levels have enacted regulations in terms of the maximum amount of such heavy metals that are generally permitted within materials that have the potential for human consumption. Thus, it is important to develop methods for minimizing the amount of such heavy metals within materials that may be utilized and ingested by humans (and other mammals).
Included within such materials are spray- or flash-dried metal silicates, precipitated silicas, silica gels, silicates, and other products that are made from silicon dioxide sources. End use formulations include many different cleaning preparations, such as dentifrices, cosmetic compositions, such as body powders, and the like, and other like applications, such as anti-caking and/or free-flowing agents that necessarily require human consumption or food contact, such as paper, plastic and rubber fillers, and pharmaceutical excipients for proper utilization thereof. Such materials that form much of the basis of these end use formulations are produced from silicon dioxide sources of various levels of heavy metal contamination. The most expensive silicon dioxide sources are considered the most pure in terms of such levels of heavy metal presence and thus generally do not require any modification to reduce potential toxic effects that may result therefrom. However, as the necessity for lower costs for source materials increases, and/or the source of pure silicon dioxide materials becomes rarer, it has thus become important to provide some manner of reducing the amount of heavy metals present within either such starting materials or, more particularly, within intermediate materials produced therefrom.
Precipitated silicas are basically produced via the initial production of metal silicates (such as, without limitation, sodium silicate) and then exposing such a material to a mineral acid (sulfuric acid, as one example), and subsequently precipitating the resultant silica produced thereby. Metal silicates, such as calcium silicate and sodium aluminosilicate, can be produced via initial sodium silicate production and addition of a metal species, such as calcium hydroxide, and an acid species to the reaction mixture to produce for instance, calcium silicate. Silica gels may also be formed through reaction of sodium silicate with acid under different conditions. In essence, it has been realized that the ability to reduce the amount of heavy metals within such ultimate products may be addressed during one of the process steps for producing the needed metal silicate intermediate material.
There have been many discussions of technologies for the removal of heavy metals, most prominently lead, from various waste materials and soils, mainly due to contamination through lead leakage or disposal. Such treatments, which include various methods including, generally speaking, thermal, biological, and physical and/or chemical treatments, have proven relatively effective for waste materials (sludges) and contaminated grounds (soils); however, nothing has been presented within the prior art that concerns the ability to reduce levels of heavy metals within specific silicate products, and certainly not during a metal silicate production method. Such prior techniques generally require soil removal removing the contaminated soil, treating it, and either replacing it on-site or disposing of it away from the area of contamination. Further developments for such decontamination of soils and sludges have involved flushing of such soil and/or sludge materials with fluids for lead dissolution, followed by potentially complex processes including immobilization of the heavy metals, precipitation of the heavy metals in an insoluble form, degradation of the heavy metal-containing materials by chemical or biological techniques (such that the heavy metals are solubilized, followed by removal of the solubilized heavy metals), or attenuation of the heavy metals by addition of inert materials to the heavy metal-contaminated soil or waste. Again, these methods do not address the ability to treat silicon dioxide materials during a silicate-producing process, and, as alluded to above, are rather complex in the function.
Phosphate materials have been suggested as possible additives to remove certain heavy metals, such as lead, from aqueous solutions, but not from solid particles or during the formation of such solid particles. Therefore, although use of phosphate minerals for immobilizing lead has been suggested in the prior art, there has been no prior disclosure of a method for accomplishing immobilization of lead or other heavy metals from either formed silicate or within a slurry from which such silicate materials are formed using solid calcium phosphate-containing materials within a high pH range.